Final answer:
Novatian was an early Christian theologian and the leader of a schismatic sect that believed lapsi, or Christians who had renounced their faith under persecution, should not be readmitted to the church. His strict views on church purity and post-apostasy reconciliation led to the establishment of Novatianism, which was later deemed heretical.
Step-by-step explanation:
Novatian was a schismatic Christian theologian in the third century who played a significant role in the controversy over the treatment of Christians who had committed apostasy under persecution, known as the lapsi. He insisted that the lapsi should not be readmitted to communion, even after doing penance, which was in contrast to the more lenient position of the Church at the time. Novatian believed that the church must be a pure body and that serious sins could not be absolved by the church's sacraments. This strict stance led to his break with the Roman church and the establishment of a separate sect that bore his name, Novatianism.
His challenge to the authority of the Roman Bishop and his consecration as an anti-pope marked a significant division and contributed to the development of early Christian theology and ecclesiastical structure. Novatian's sect continued for several centuries, despite being labeled as heretical by the mainstream church.